A passcode is the eight-digit number that most cards have in the bottom-left corner on the card (certain cards, such as "Gate Guardian", do not have this card number, and some cards instead have Limitation Text in this spot).
Passcodes, while serving no real purpose in the actual game, do serve a purpose in video games. By entering a passcode found on a given card (usually in a "Password" selection screen), a player may obtain that card for their virtual collection, although various restrictions are placed on this depending on the game. In most games, this can only be done once per card. In some games entering the code will get the player the card for free, while in others the player must pay in-game currency to do so. In most of the latest games, players must also have to have the card they are trying to get in their card list, which either involves owning the card, or owning most of the cards in the set the card comes in.
A few games have their own passcodes for cards, separate from the passcodes printed on the physical cards; these often contain a mixture of numbers and uppercase letters instead of being just numbers.